The lungs are the most highly perfused organs in the body. Disease processes that interfere with perfusion, such as pulmonary thromboembolism can be difficult to diagnose, and are potentially lethal. The investigators have developed novel methods for perfusion MRI of the lungs. The specific aims of this research are: 1. To develop the methodology for imaging and quantification of pulmonary perfusion using dynamic first-pass imaging after administration of a bolus of gadolinium chelate. 2. To quantify perfusion by this technique and correlate the results with absolute flow as determined by microspheres in an animal model, using pharmacologic agents to modulate perfusion. 3. To optimize MR angiography for patients unable to suspend respiration for lengthy periods. 4. The determine the sensitivity and specificity of perfusion MRI and MR angiography for the diagnosis of pulmonary embolism in an animal model. To determine the time course of enhancement of perfusion defects caused by pulmonary emboli. 5. To perform a preliminary study of the MRI perfusion and MR angiography methods in patients with suspected pulmonary embolism who have high probability ventilation-perfusion scans. To use the HASTE technique to detect lung abnormalities such as infiltrates, masses or bulla that could mimic perfusion defects from a pulmonary embolus.